1. Technical Field
The present invention relates to a lithography technique, and more particularly, to a lens structure, an optical system including the same, and a lithography method using the optical system.
2. Related Art
It is essential to use a lithography process for forming a pattern to manufacture electronic devices, such as semiconductor integrated circuit devices and display devices. In such a lithography process, a photo mask, which is generally called a reticle, can be used. An image formed on the photo mask is projected onto a semiconductor substrate (a so-called wafer or glass substrate) having a photoresist material applied thereon by a projection optical system. In recent years, a step and repeat method of synchronously scanning the reticle and the wafer with light has been generally used.
The projection optical system is provided in an exposure apparatus. The resolution of the optical system increases as the wavelength of exposure light used is shortened and the numerical aperture (NA) of the projection optical system increases. Therefore, with an improvement in the degree of integration of an integrated circuit, the wavelength of exposure light used in the projection optical system has been shortened, and the numerical aperture of the projection optical system has increased. In recent years, a KrF excimer laser having a wavelength of 248 nm or an ArF excimer laser having a wavelength of 193 nm has been generally used as an exposure light source.
In the lithography process, the most important factors are resolution and the depth of focus (DOF). The resolution (R) and the depth of focus (δ) are calculated by Expressions 1 and 2 given below:R=k1·λ/NA, and  [Expression 1]δ=k2·λ/NA2  [Expression 2](where λ indicates the wavelength of exposure light, NA indicates the numerical aperture of the projection optical system, and k1 and k2 indicate process coefficients). According to Expressions 1 and 2, a trade off relationship is established between the resolution (R) and the depth of focus (δ). That is, when a light source emitting exposure light having a short wavelength λ and a projection optical system having a high numerical aperture are used in order to increase the resolution, the depth of focus (δ) decreases. In order to ensure a constant depth of focus (δ), an auto focus method is used to align the surface of a wafer with an image surface of the projection optical system, when the exposure processing is performed. However, in this case, it is difficult to obtain a sufficient depth of focus (δ). Therefore, various methods, such as a phase shift reticle method, an off-axis illumination method, and a multi-layer resist method, have been proposed in order to increase the depth of focus.
As described above, in the projection optical system according to the related art, the depth of focus has decreased with a reduction in the wavelength of exposure light and a widening of aperture of the projection optical system. In addition, it is expected that the wavelength of exposure light will be shortened as the degree of integration of an integrated circuit is improved in the near future. In this case, the depth of focus will be excessively decreased, which may cause an insufficient margin during an exposure operation.
Therefore, Korean Patent Laid-Open No. 10-2005-0001086 discloses “Two-dimensional light-modulating nano/micro aperture array and high-speed nano pattern recording system using the array” using an evanescent field in order to improve the resolution and lithography characteristics, which is filed by the applicant. In the two-dimensional light-modulating nano/micro aperture array, an aperture serves as a photo mask, and light focused through a fine aperture is transmitted to a recording medium (i.e., substrate) that is spaced from the aperture by a predetermined distance, thereby transferring an image onto the recording medium.
The two-dimensional light-modulating nano/micro aperture array uses an evanescent field to improve the resolution and performs exposure with the depth of focus being increased, which makes it possible to improve reliability. However, it is difficult to maintain a constant distance between the two-dimensional light-modulating nano/micro aperture array and a recording medium (i.e., substrate), that is, a constant focal distance. When the distance between the two-dimensional light-modulating nano/micro aperture array and the recording medium is not kept constant, a different exposure pattern may be formed even when light having the same intensity is incident.